This invention relates generally to reactive binding material for use with cementitious building materials, and more specifically to the mechanical activation of granitic quarry fines into a pozzolan-type material.
The stockpiling and disposal of fines produced from aggregate quarrying and crushing operations is one of the major problems now facing the United States"" aggregate industry. (International Center for Aggregates Research, xe2x80x9cAn Investigation of the Status of By-Product Fines in the United States,xe2x80x9d Research Report ICAR 101-1, 1997.) To date, research has generally focused on developing techniques for adding fines as a non-reactive filler to concrete and asphalt products. Other limited studies have evaluated fines for use as soil admixtures and amendments, mineral fillers, or for use as flowable fill material. Unfortunately, none of these applications has been particularly successful, and so these waste fines must be handled, stockpiled, and ultimately storedxe2x80x94consuming large quantities of energy and real estate, without benefit to the quarry operator or to the public.
Little research, however, has been devoted to chemically changing the quarry fines to alter their reactive characteristics. xe2x80x9cMechanochemical processingxe2x80x9d uses mechanical energy to activate or accelerate chemical reactions and/or cause changes in chemical structure. Most research today is performed in the area of bimolecular xe2x80x9cmechanically alloyingxe2x80x9d where the goal is to produce new or novel microstructures with enhanced mechanical properties from two or more starting materials. In unimolecular reaction systems, most of the research in mechanical activation has been done on single-phase material, such as silica sand, rather than on multi-mineral systems, such as granite. Interestingly, a recent review article (P. G. McCormich and F. H. Froes, xe2x80x9cThe Fundamentals of Mechanochemical Processing,xe2x80x9d J. of Materials (November 1998), p. 61) mentions that the application of mechanochemical methods to waste processing xe2x80x9chas not yet received much attention, but is an area worthy of exploration.xe2x80x9d
The construction industry uses a wide variety and large quantity of mineral-based materials in the manufacture of concrete structures, precast concrete products, mortars, and similar products. Processes used to create such mineral-based building materials vary from relatively complex to quite simple. For example, the manufacture of portland cement includes mining and crushing limestone, mixing in other minerals, calcining the limestone mixture in a kiln to form clinker, and then mixing the clinker with gypsum before grinding it into cement. In contrast, producing aggregate for use in concrete or as a road base is less complicatedxe2x80x94rocks are typically quarried, crushed, and sieved to sort the desired size(s) of aggregate from the crushed rock. Another simple process is the mining of clean sand for use as a filler in mortar or concrete.
Because building materials and components for building materials typically are relatively heavy and located in different geographical locations, transporting the materials to a worksite can be expensive. An example of a material where transportation costs significantly impact use is fly ash. Fly ash, a type of pozzolan, is a byproduct of coal-burning power plants, and can be used as a reactive, cementitious material in the production of hydraulic cement. A typical cement mixture may have up to 20% of the portland cement content replaced by fly ash. Under certain circumstances, using fly ash instead of portland cement results in a more economical and durable concrete mix. Fly ash is relatively inexpensive at the power plant source, presently costing only about $3-4 per ton. This compares to about $80-90 per ton for the portland cement which the fly ash replaces in the concrete. Transporting fly ash from the Midwest, where it is often produced, to the West, where there is a demand for its use, increases the cost of the fly ash to about $40-50 per ton. Therefore, it would be desirable to provide a fly ash substitute that can be produced closer to the point of use and provides adequate strength in a concrete or mortar mixture.
A method is presented here to increase the reactivity of quarry fines by changing their chemical state utilizing a mechanical process, enabling their use as a partial replacement for portland cement.
Granitic quarry fines that would otherwise be a waste byproduct of a quarry operation can be mechanically activated to become a cementitious or reactive component of concrete and/or similar materials including ceramics and/or polymers. The fines are milled, typically by a high-energy milling process such as in an attritor mill, to convert the fines from an essentially non-reactive state into a reactive state. Like fly ash pozzolans, the granitic-based pozzolan chemically combines with calcium from the hydration of portland cement, when the pozzolan and cement are mixed with water and cured. The use of a suitable mill, such as an attritor mill, results in a pozzolan which has mechanically activated particles. The effectiveness of the mechanical activation can be determined by compressive strength measurements. Generally, the process provides a pozzolan which has mechanically activated particles, wherein particles having a maximum dimension of about 10 microns in the sample population and contain numerous macroparticles formed from an agglomeration of microparticles. The pozzolan, like fly ash, reacts with lime (calcium hydroxide) to form hydrated calcium silicate product. In another embodiment the invention provides a cement-based material which includes a portland cement paste hydrating in concert with activated granitic-based pozzolan.